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Risk Management Activities - Impact of hedged items designated in hedging relationships on OCI and net earnings (Details) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disclosure of detailed information about hedging instruments [line items] |  |  |
Gains (losses) on cash flow hedges, before tax | $ 28 | $ 77 | $ (9)
Gains (losses) recognized in other comprehensive income | (141) | (53) | (57)
Gain (loss) on hedge ineffectiveness recognised in profit or loss | 0 | 0 | 0
Commodity contracts |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Gains (losses) on cash flow hedges, before tax | 41 | 77 | (9)
Commodity contracts | Revenue |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Gains (losses) recognized in other comprehensive income | (137) | (59) | (67)
Gain (loss) on hedge ineffectiveness recognised in profit or loss | 0 | 0 | 0
Foreign exchange forwards on project hedges |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Gains (losses) on cash flow hedges, before tax | (1) |  |
Foreign exchange forwards on project hedges | Property, plant and equipment |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Gains (losses) recognized in other comprehensive income | 0 |  |
Gain (loss) on hedge ineffectiveness recognised in profit or loss | 0 |  |
Foreign exchange forwards on US debt |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Gains (losses) on cash flow hedges, before tax |  |  | 0
Foreign exchange forwards on US debt | Foreign exchange (gain) loss |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Gains (losses) recognized in other comprehensive income |  |  | 3
Gain (loss) on hedge ineffectiveness recognised in profit or loss |  |  | 0
Forward starting interest rate swaps |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Gains (losses) on cash flow hedges, before tax | (12) | 0 | 0
Forward starting interest rate swaps | Interest expense |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |
Gains (losses) recognized in other comprehensive income | (4) | 6 | 7
Gain (loss) on hedge ineffectiveness recognised in profit or loss | $ 0 | $ 0 | $ 0

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